Industrial plants often include power consuming devices such as, for example, electric motors, pumps, compressors and/or HVAC systems. These devices are often referred to as loads. Many industrial plants control the loads of their facility with electrical power converters to improve process control and increase energy efficiency such as, for example through the regulation of variable speed devices and the minimization of inefficient power consumption. Power converters typically behave as non-linear loads. A non-linear load draws distorted input current at multiple frequencies from the electrical power source, whether that source is supplied by a utility or a local generator.
As seen in FIG. 1, distorted currents 14 are currents that have at least a fundamental component 10 and a harmonic component 12. The fundamental component 10 delivers the energy for the load to do useful work. Although necessary for non-linear loads, the harmonic component 12 of the current(s) 14 performs no useful work.
The harmonic component 12 is harmful to utility transformers, local generators and other electric loads on the same electric supply as they cause excessive heating, voltage distortion on the electrical supply and potentially impact operation of other equipment sharing the power source.
In order or keep the harmonic component drawn from a source at a safe level, electric utilities and end users are adopting uniform power quality standards such as IEEE-519. One way electric utilities and end users are complying with uniform power quality standards is by using harmonic filters to locally source the harmonic component needed by the non-linear load. If a harmonic filter supplies the harmonic component required by the non-linear load, the harmonic component supplied by the power source is significantly reduced or eliminated.
One type of filter being used to comply with uniform power quality standards is an electronic active filter. Electronic active filters effectively act as a local harmonic component source to supply the necessary harmonic component to non-linear loads. Such electronic active filters have been typically used as a shunt as shown in FIG. 2. The electronic active filter 16 operates as a shunt connected current source by creating an output current, IAF 18, for supplying the harmonic component 12 for the non-linear load(s) 20. In this arrangement, the electronic active filter 16 produces the harmonic current 12 drawn by the non-linear load(s) 20, eliminating a harmonic component from being drawn from the source 22. As a result, the source 22 supplies a source current 24, via Isource, containing the fundamental current 10 in accordance with the uniform power quality standards.
Shunt electronic active filters generally have two main structures, a power circuit 26 and an independent control system 28, as seen in FIG. 3. The power circuit 26 is used to produce the harmonic component 12 and inject the harmonic component into the electrical system. The independent control system 28 is used to determine what harmonic component 12 should be produced, referred to as current reference, and control the power circuit to accurately produce and track the current reference(s). The shunt electronic active filter could also be used to produce a volt-ampere reactive.
As seen in FIG. 3, the independent control system 28 generally consists of an outer loop regulator 30, an inner current regulator 32 and a voltage modulator 34. The outer loop regulator 30 receives the current(s) 14 of the electrical system desired to be filtered. Based upon the current(s) 14, the outer loop regulator 30 generates a filter reference signal for the current, that is, current reference 36. The inner current regulator 32 receives the current reference 36 as well as feedback measurements 39 of the output of the electronic active filter 16. Based upon the current reference 36 and feedback measurement 39, the inner current regulator 32 generates a voltage reference 38. The voltage modulator 34 receives the voltage reference 38 and converts that voltage reference 38 to gate signals 40 that are output to the power circuit 26.
FIG. 4 shows the schematic of an exemplar electronic active filter power circuit 26. The illustrated power circuit 26 is capable of injecting 3-phase harmonic currents (e.g. IAF_A, IAF_B, IAF_C) into a 3-phase electrical system; however other power circuits are known in the industry and the use of such power circuits would not depart from the spirit of the invention.
The illustrated power circuit 26 contains a two level DC to AC power converter 42 consisting of the DC bus capacitor, CDC 44, and six power electronic switches, Q1-6 collectively 46. The switches 46 can be of any type, but are shown for explanatory purpose as IGBTs. The IGBTs shown are controlled by gate signals to turn on and turn off at switching frequencies higher than the frequency of the electrical system's fundamental component 10, as determined by the independent control system 28, to produce voltages Vpole_A, Vpole_B, Vpole_C.
A three-phase low pass LCL filter (e.g. L1, C1, L2) 47 converts each of the voltages Vpole_A, Vpole_B, Vpole_C, into the three-phase output currents (e.g. IAF_A, IAF_B, IAF_C). The filter 47 locally filters out extraneous or unwanted currents, such as the high frequency switching ripple current, but allows the lower frequency harmonic currents to pass into the electrical system. The control system 28 determines the pattern of IGBT gate signals (GQ1-GQ6) 40 that most accurately produce the necessary harmonic component 12 in the active filter output current 18.
The current(s) 14 of the electrical system desired to be filtered can be determined and supplied to the outer loop regulator 30 of the independent control system 28 in a number of different ways. The two most common ways for a single, e.g. non-paralleled, electronic active filter to obtain the current(s) 14 of the electrical system desired to be filtered are load side sensing and line side sensing.
Load side sensing is an open loop control method in which the load current (ILoad) is directly or indirectly sensed. FIG. 5 shows an example of direct sensing of the load side. The load current(s) 14, ILoad, is sensed for example, by a current sensor 50. Although a current sensor is described, the term is intended in a broad sense, and a number of devices are known in the industry to sense current, e.g. a transducer. The sensed current(s) 48 of the load current(s) 14, broadly defined as the sensed current itself or at least a signal representing or indicating that current or the level or value of that current or a component of that current, is received by the outer loop regulator 30. The outer loop regulator 30 extracts the fundamental component 10 from the sensed current(s) 48. The extraction of fundamental component 10 can be done by a high pass filter although other devices are known in the industry. The fundamental component 10 can be determined by a number of methods known in the industry such as an adaptive notch filter with a phase lock loop to determine the notch frequency.
After the fundamental component 10 is stripped from the sensed current(s) 48, the harmonic component 12 of the sensed current 48 is used to output a current reference 36 to the inner current regulator 32. The filter output current 18, e.g. IAF, of the power circuit 26 is sensed for example, by a current sensor 52, and provided to the inner current regulator 32. Here again, the output of the current sensor 52 is broadly defined as the sensed current itself, a component thereof or at least a signal representing or indicating that current or the level or value of that current. A summation junction 54 of the inner current regulator 32 compares the current reference 36 to the sensed current feedback 39 to determine a comparison or error 56 which is sent to a compensator 58, G, such as for example via a comparison signal. The inner current regulator 32 is represented in FIG. 5 as a standard closed loop regulator although other methods for regulating the power circuit are known and used in the industry. The compensator 58 processes the error 56 and outputs a voltage reference 38. The voltage modulator 34 receives the voltage reference 38 and, based on that voltage reference, outputs gate signals 40 to the power circuit 26. Power circuit 26 thereby outputs a current 18 to the electrical system as described above. From the point where the current reference 36 is output to the inner current regulator 32, to the point where a current 18 is output by the power circuit 26, is indicated as a dashed box 60, which will be referred to as the inner electronic active filter 60. The device enclosed by dashed box 61 will herein be referred to as the load side electronic active filter 61.
The compensator 58 could be designed for example, to meet current tracking performance metrics. A couple of exemplary or common compensator implementations include proportional; proportional and integral; and proportional, integral and differential compensators. Other implementations are known in the industry and could also be used without departing from the spirit of the invention. The harmonic component demand of the load current(s) 14 is supplied by the electronic active filter 61, thus eliminating the harmonic components from being supplied from the source 22.
Load side sensing can be beneficial because it can be relatively straight forward to implement in state of the art power converter controllers and because multiple active filters can be paralleled using this control method to reach higher current levels as described further below. However, load side sensing is an open loop control method which has inherent inaccuracies and is sensitive to open loop errors. For example, any errors in the current sensors 50, 52 or in the implementation of the inner current regulator 32 can lead to current regulator tracking errors and remnant harmonic currents in the source 22. Also, the physical installation of load side sensors can be difficult in certain applications, such as motor control centers where the load electrical bus is not easily accessible, or where multiple non-linear loads are present.
Line side sensing is an alternate method that overcomes many of the problems associated with load side sensing. As shown in FIG. 6, line side sensing is a closed loop control method wherein the sensed current(s) 48 of the source current 24, ISource, is sensed for example, by a current sensor 50. The voltage could also be sensed, for example, in order to determine the fundamental frequency. Additional electrical system quantities could also be sensed with addition sensors. Because line side sensing is a closed loop control method, it is not as sensitive to open loop errors as is load side sensing and can yield better performance due to the closed loop control action. Further, line side sensing is usually easier to install because the AC voltage source bus in a facility is often more accessible for installing current sensors. Line side sensing also provides filtering for all non-linear loads present.
Once the current(s) of the source current 24 is sensed, the sensed current(s) 48 is sent to a filter controller 62. The filter controller 62 removes the fundamental component 10 and outputs the harmonic component 12 as a feedback 64 to the outer loop regulator 30.
In addition to receiving the harmonic component feedback 64 of the source current 24, the outer loop regulator 30 also receives a filter reference 66. Because it is desired in this illustrated example, that the source 22 supply no harmonic component 12, the filter reference 66 is set to zero. The summation junction 68 of the outer loop regulator 30 compares the harmonic component feedback 64 to the filter reference 66 to determine a comparison or error 70 which is sent to a compensator 71, G1, such as for example via a comparison signal. The compensator 71 processes the error 70 and outputs a current reference 36. Due to the closed loop action, the outer loop regulator 30 outputs an often-adjusted current reference 36 to drive down the harmonic component feedback 64 being supplied by the source 22. At steady state, the current reference 36 is equal to the harmonic component 12 drawn by the non-linear load 20. Once current reference 36 is output, the inner electronic active filter 60 operates as previously described with reference to FIG. 5. Although the prior art circuit shown in FIG. 5 is shown and described using a filter reference 66, other means for generating an error 70 are known and used in the industry, including using no harmonic reference at all. From the point at which a feedback 64 is supplied to the outer loop regulator 30 up through the point that a current 18 is output by the power circuit 26 will be referred to as the line side electronic active filter 72.
Generally electronic active filters are rated based on their output current capacity. The necessary capacity of the electronic active filter(s) is based on the amount of harmonic component 12 in the load current(s) 14. In many applications, the amount of harmonic correction current needed to eliminate harmonic current from the source 22 exceeds the capacity of a single electronic active filter. In these cases, multiple electronic active filters with independent control systems are deployed in parallel using a combination of the line side and load side sensing.
FIG. 7 shows an example of parallel electronic active filters wherein all the electronic active filters are load line sensing. Because load side sensing is an open loop control method, as referred to above, multiple electronic active filters can be placed in parallel. FIG. 7 illustrates an exemplary embodiment wherein two load side electronic active filters 61, 61′ are shown. A current sensor 50 senses the load current(s) 14 and outputs the sensed current 48 to both load side electronic active filters 61, 61′. Before the sensed current 48 is received by the outer current regulators of the load side electronic active filters 61, 61′, the sensed current 48 is divided by the number of load side electronic active filters. Therefore, in a system with N parallel load side electronic active filters, each load side electronic active filter will operate on 1/Nth of the sensed current(s) 48 of the load current(s) 14 and supply to the electrical system via its harmonic component output 18 1/Nth of the harmonic component 12 drawn by the non-linear load 20. The example illustrated in FIG. 7 is performed entirely using an open loop control method and therefore, as described above, has the inherent performance limitations of a single open loop active filter control method described above and, in fact, would be compounded based on the use of additional load side electronic active filters.
Another example of parallel electronic active filters is shown in FIG. 8. The example illustrated in FIG. 8 has one line side electronic active filter 72 and one load side electronic active filter 61. However, any number of load side sensing electronic active filters could be added because, as described above, load side sensing is an open loop control method and there is no conflict. In the embodiment shown in FIG. 8, a current sensor 50 senses the source current 24 and outputs the sensed current 48 to the fundamental extractor or filter controller 62 of the line side electronic active filter 72. Thereafter, line side electronic active filter 72 operates as described above. Another current sensor 50′ senses the load current(s) 14 and outputs the sensed current 48′ to the load side electronic active filter 61. Thereafter, the load side electronic active filter 61 operates as described above. Although the example illustrated in FIG. 8 is not performed entirely using an open loop control method, it is partially open loop, and to that extent still has the inherent performance limitations of a single open loop active filter described above.
Yet another example of parallel electronic active filters is shown in FIG. 9, in which the load current(s) 14 is synthesized. This arrangement is used when the load bus is inaccessible for load side sensing. In this embodiment, the current sensor 50 outputs the sensed current 48 of the source current 24 to a summing junction 76 and also to a fundamental extractor or filter controller 62 of the line side electronic active filter 72. Thereafter, line side electronic active filter 72 acts as previously described above. A current sensor 75 senses the sum current 74 of the currents 18, 18′ being output by the electronic active filters 72, 61 respectively. The sum current 74 is output to the summation junction 76 and is compared to the sensed current(s) 48, the result of which is called the synthesized load current 78. Summing junction 76 could be, for example, a current sensor, or the function could be accomplished by a microprocessor. The synthesized load current 78 is sent to the load side electronic active filter 61, which operates as described above. In the example illustrated in FIG. 9, the total harmonic component or sum current 74 is measured directly with one current sensor 75; however, the sum current 74 could be determined by using a separate current sensor, e.g. 52, 52′ to sense each output 18, 18′ and sum the harmonic components such as, for example, by a summing junction. Although the example illustrated in FIG. 9 has a line side electronic active filter 72, it still has the inherent performance limitations of a single open loop due to the load side electronic active filter 61 being, as described above, set up in an open loop configuration.
Paralleling line side electronic active filters is not currently known, because any arrangement now known would result in uncontrolled and unacceptable circulating currents between filters, thereby reducing performance. A circulating current between electronic active filters is current that flows between filters and but does not cancel the load harmonic component being drawn from the source. Because each electronic active filter has a maximum current it is capable of producing, the additional circulating current reduces the current available to supply the harmonic component being drawn by the non-linear load, thereby allowing the harmonic component to be drawn from the source. As is seen from the examples provided herein, therefore, currently all paralleling schemes for multiple electronic active filters require some or all of the electronic active filters be configured in a load side sensing arrangement, which, as described further above, has inherent performance drawbacks.
As a result, there exists a need to parallel all electronic active filters in a line side sensing arrangement to capture the performance benefits of the closed loop control method described above, while still avoiding unacceptable circulating currents.
It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can lead to certain other objectives. Other objects, features, benefits and advantages of the present invention will be apparent in this summary and descriptions of the disclosed embodiment, and will be readily apparent to those skilled in the art. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying figures and all reasonable inferences to be drawn therefrom.